[CentOS] C7: How to configure raid at install time

Fri Nov 20 04:02:18 UTC 2015
Warren Young <wyml at etr-usa.com>

On Nov 19, 2015, at 3:49 PM, Devin Reade <gdr at gno.org> wrote:
> size of the disk was never the original motivation for
> keeping / separate, at least within my memory

Prior to SysV, the location of user home directories was not standardized, and AT&T recommended that you put them in /usr.[1]

Also, in the PDP days, you had things like the RL and RK series drives, which started at 2.5 MB.  If you were well-heeled, you might have several of these, getting you up into the tens of megs.  Thus, it made sense to put one major filesystem on each disk, like /, /usr, and /var for a 3-drive system.

> it was to minimize the
> amount of disk space that needed to be fsck'd before bringing the system
> to single user mode

That may have been a happy side benefit, but I’ve never read such a thing in a quarter century of using Unix.


> Even in the days of
> SunOS 3 (that's SunOS, not Solaris) I was installing the entire OS on
> one physical drive, partitioned.

Unix goes back about 15 years before the advent of the Sun-3.  Being fairly high-end boxes of the time, they likely would have had HDDs in the hundreds of megs range, possibly even the single-digit gigs, plenty for a complete 4.2BSD install.

This was also the time in computing history where diskless or small-disk workstations were common, which gives another reason to make /usr separate: get an NFS stack up via RARP or the small local system drive, then attach to a server’s disk to complete the boot process.

> The main message is that while CentOS 6 and before *could* have
> / and /usr on different filesystems, don't do it with CentOS 7
> per <https://access.redhat.com/solutions/53005> (paywalled).

I don’t need to pierce the paywall to know it’s a bad idea.  This is enough of a clue:

    $ ls -l /bin
    lrwxrwxrwx. 1 root root 7 Jul  7  2014 /bin -> usr/bin

[1]: See page 4-8 in the SVR3.2 sysadmin’s guide: http://goo.gl/E9quko